1. Field of the Invention
The present invention relates generally to mass finishing methods and machines used to process and to remove material from surfaces of workpieces such as surface finishing of metal workpieces to produce either a surface ready for further treatment (e.g., plating or coating) or a finished article.
2. Description of Related Art
Mass finishing is an industrial process used in various forms to remove material such as, but not limited to, metal from the surface of workpieces. The material is removed by physically contacting the objects with a collection of solid abrasive particles with or without the use of additional chemicals to enhance the process. Mass finishing is used in various forms to smooth-out and to reduce the surface roughness of workpieces. Mass finishing processes are used for finishing the surfaces of objects, particularly, metal workpieces, by physically contacting the objects with a collection of solid particles with or without the use of additional chemicals to enhance the process. Mass finishing machines and methods usually incorporate a motor-driven vibrating container that holds a specialized media along with appropriate chemicals or water that enhance the finishing capability and action of the media.
Mass finishing is used to reduce or eliminate the amount of hand finishing that must be applied to metal articles including articles that are processed for functionality, for example, airplane propellers which are subsequently painted. More examples of such articles include, articles that are to be further plated, for example, with chromium plating such as is commonly used in open-style alloy wheels. Cast wheels have a coarse surface that requires appropriate treatment either to give a final appearance or to prepare the surface for further plating or coating processes. These wheels tend to be difficult to completely finish in conventional media finishing machines before they are suitable either for sale as finished or after plating, painting, or other coating.
Drag finishing has been used for mass finishing. However, the workpieces being processed tend to leave the equivalent of a wake behind them as they are dragged through the media, typically, in a revolving circular pattern. The skewed pattern of the media in the wake keeps the media from presenting itself to the full face of the object being polished (or vice versa) thus, either reducing the quality of the finished part or greatly extending the time required to carry out the finish, or both. It also causes much more power to be used.
Chrome-plated wheels formed of aluminum or alloys are typically difficult to finish because of the openings (“windows”) in the wheels which often require much hand finishing. If the wheels are intended to be chrome plated, they tend to take the plating less favorably in the “low current density” area of the wheel. As a result, the chrome plating process can magnify, rather than reduce, the coarse or rough appearance in those areas.
Coarse surfaces require appropriate treatment either to give a final appearance or to prepare the surface for further plating or coating processes. These wheels tend to be difficult to completely finish in conventional media finishing machines, however, usually require several steps of hand polishing and buffing, both before and after any media finishing before they are suitable either for sale as finished or after plating, painting, or other coating. This is labor intensive, time consuming, and expensive.
During finishing, the media wears down with use, the abrasive particles of the media become smaller and smaller until it is just abrasive dust. With wear, pounds of media are lost and make-up must be added. Over time, some of the media is virtually new and as supplied in size, some is just about to be completely consumed, resulting in a steady-state condition where the abrasive media is present in any size.
A typical drag finisher uses a large horsepower drive (typically in a range of 50-100 Hp) for tub rotation and a secondary smaller horsepower drive(s) (a few Hp per station) for customer part rotation about a spindle axis. The media is subject to a water/soap mist of water and spent abrasive media. In order to keep the spent media from fouling the system between processing cycles, the vessel is stopped and vibrated by means of an internally mounted eccentric rotation mass to roll the media in the vessel to counter the natural stratification, and to help drain the resulting slurry from the floor of the vessel through grated openings.
When finishing workpieces in a typical drag finisher, tub rotation drives the media to dam-up in front of the workpiece. This is the primary source of resistance to tub or vessel rotation and, in turn, the primary cause of inefficiency of the current and typical processes. This resistance results from the pressure created when the abrasive media does not flow over and through the workpiece. The media locks and balls up on itself and is pushed around in a circle consuming lots of energy not applied to finishing the workpiece, but wearing against itself and doing little work to the workpiece or the customer's part. The process exhibits a pressure bias towards the surfaces deeper down into the media. Rotation about the spindle axis presents all surfaces on the workpiece equally to the biased media pressure.
As a result of the balling-up, little media finds its way past the workpiece and through the openings in the workpiece and, as a result, the process has a slow gain in improvement of the surface roughness per unit of run time. This is not very economical and reduces throughput. Secondly, the outer edges of the workpiece are cut disproportional and the workpiece loses definition and is cut undersized, while some of the interior faces are not cut enough. Vessel or tub rotation about its axis is such that a tangential media velocity is in the 300-400 feet per minute, measured at the imaginary circle through the center of the workpiece. Workpiece rotation about the spindle axis is in the 5 to 20 rpm range.
If the mass finishing process fails to remove sufficient material or otherwise fails to properly polish the surface, the workpiece generally must be hand finished with small finishing tools. The hand finishing process tends to be labor intensive, relatively slow, and generally expensive. Additionally, the hand finishing will discharge unwanted metals into the ambient surroundings.
Accordingly, a need exists for a mass finishing technique that can successfully and completely finish all of the custom and difficult-shaped portions of certain object such as automobile wheels and do so in a manner that either successfully supports later plating or coating, or that produces a finished workpiece that has little or no need of hand finishing (or of other mechanical finishing such as relatively expensive robotic belting or buffing machines) prior to marketing and use.